Because they are fundamentally different, they are both software centers that is about all they have in common.
Newer software centers rely on xml data that must be added to each and every package to even appear in the software center... I hate it. But that how all the big software centers function and we do not have the resources or interest in rewriting, improving and maintaining solus-sc.
We talk about a task to add more to the newer software centers every week with this task in the Solus Cleanup Crew:
An that is focusing on GUI applications not CLI ones or libraries etc, there are many many more to add.